1. Field of the Invention
The present invention relates to optical systems. More specifically, the present invention relates to systems and methods for interferometric testing of optical systems.
2. Description of the Related Art
The precision to which an optical surface can be fabricated is largely driven by how precisely it can be tested. The advent of the laser and phase-measuring interferometry has given fabricators powerful tools to measure, for example, optical surfaces to within a few nanometers departure from the theoretically perfect optical surface. Although the use of interferometry has made the testing of flats and spherical surfaces relatively simple, interferometric testing of optical systems, i.e., end-to-end testing of optical systems, represents a problem of increased complexity. For example, interferometric testing of optical systems is usually performed from the object side of the optical system to the image side. To achieve end-to-end testing, a retro-reflective ball is typically placed in the image plane so that the light traversing the optical system under test is reflected from the surface of the ball and returns to the interferometer. In many cases, it would be desirable to test the optical system from the image side rather than the object side.
It will be noted that the testing of an optical system does not rely on the results of a single test performed with respect to a single image point. Optical system testing necessitates testing with respect to a number of image points. There are generally two techniques for generating these image points. The first is to physically lock the interferometer and optical device generating the point with respect to one another and then to move the interferometer-optical device combination to a series of predetermined locations relative to the optical system under test. One major drawback with this technique is that the retro-reflector on the object side of the optical system under test must be realigned, at least in tip and tilt, for each predetermined location in the test sequence. Alternatively, an array of lenses can be employed to generate a plurality of points. One major problem with the latter technique is that the plurality of lenses needed to generate the multiple point sources physically interfere with one another, e.g., overlap, in fast systems.
What is needed is a system which facilitates testing of an optical system from the image side. More specifically, what is needed is a plane reflector capable of retro-reflecting multiple point sources of light to facilitate multipath testing of an optical system using an interferometer disposed in image space. Most specifically, what is needed is a multiple plane reference mirror for reflecting multiple point sources back along the angle of incidence for testing of the optical system using an interferometer located in image space. Ideally, the multiple plane reference mirror would be a thin, unitary structure, which would permit optimization of the interferometric testing system with respect to, for example, size and weight.
The need in the art is addressed by a multiple plane reference mirror for an interferometric testing system and a corresponding method for operating such as system. The interferometric testing system including the multiple plane reference mirror of the present invention advantageously provides an interferometric testing system which minimizes the need for system realignment between testing events with respect to individual point sources generated by repositioning the interferometer with respect to the optical system under test. The need in the art is also addressed by a method of testing an optical system using the multiple plane reference mirror.
Preferably, an interferometric system for testing an optical system includes an interferometer which outputs a light beam and analyzes the returned light beam, a lens which forms a perfect point in an image plane of the optical system responsive to the light beam, and a multiple plane reference mirror which reflects the light beam corresponding to the image point transmitted by the optical system back through the optical system into the interferometer for analysis. According to another aspect of the invention, the multiple plane reference mirror comprises a holographic multiple plane reference mirror.
Alternatively, the interferometric testing system for testing an optical system according to the present invention includes a first device for outputting a light beam, a second device for analyzing a selected light beam, a third device for generating an image point in an image plane of the optical system responsive to the light beam, and a fourth device for reflecting the light beam corresponding to the image point transmitted by the optical system back through the optical system to thereby generate the selected light beam. Advantageously, the fourth device reflects the light beam back along the arrival path of the light beam. Preferably, the fourth device is a multiple plane reference mirror and, most preferably, the multiple plane reference mirror is a holographic multiple plane reference mirror.
The above-mentioned interferometric testing system including a multiple plane reference mirror can be operated to thereby evaluate an optical system under test by steps for generating an image point at a selected one of N predetermined test points, passing light corresponding to the image point from the image side of the optical system under test to the object side, reflecting light corresponding to the image point from the object side of the optical system under test to the image side, collecting data corresponding to the reflected light, repeating the generating, passing, reflecting, and collecting steps for each of the N predetermined test points, and analyzing the collected data for all of the N predetermined test points.